In the recent technique for removal of ionic substances contained in seawater, brackish water or the like, separation methods utilizing separation membrane elements have found increasing uses as processes for energy savings and conservation of resources. Separation membranes adopted in the separation methods utilizing separation membrane elements are classified into five groups according to their pore sizes and separation performance, namely microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, reverse osmosis membranes and forward osmosis membranes. These membranes have been used, for example, in production of drinkable water from seawater, brackish water, water containing deleterious substances, or the like, production of ultrapure water for industrial uses, effluent treatment, recovery of valuable substances, or the like, and have been used properly according to ingredients targeted for separation and separation performance requirements.
Separation membrane elements have various shapes, but they are common in that they feed raw water to one surface of a separation membrane and obtain a permeated fluid from the other surface thereof. By having a plurality of separation membranes tied in a bundle, each separation membrane element is configured to extend the membrane area per separation membrane element, in other words, to increase the amount of a permeated fluid obtained per separation membrane element. Various types of shapes, such as a spiral type, a hollow fiber type, a plate-and-frame type, a rotating flat-membrane type and a flat-membrane integration type, have been proposed for separation membrane elements, according to their uses and purposes.
For example, spiral-type separation membrane elements have been widely used in reverse osmosis filtration. The spiral-type separation membrane element is provided with a central tube and a stack wound up around the central tube. The stack is formed by stacking a feed-side channel material for feeding raw water (that is, water to be treated) to a surface of a separation membrane, a separation membrane for separating ingredients contained in the raw water and a permeate-side channel material for leading into the central tube a permeate-side fluid having been separated from the feed-side fluid by passing through the separation membrane. In the spiral-type separation membrane element, it is possible to apply pressure to the raw water, and therefore, it has been preferably used in that a larger amount of a permeated fluid can be taken out.
In the spiral-type separation membrane element, generally, a net made of a polymer is mainly used as the feed-side channel material in order to form a flow channel for the feed-side fluid. In addition, a multilayer-type separation membrane is used as the separation membrane. The multilayer-type separation membrane is a separation membrane provided with a separation functional layer formed of a crosslinked polymer such as polyamide, a porous resin layer (porous supporting layer) formed of a polymer such as polysulfone, and a nonwoven fabric substrate made of a polymer such as polyethylene terephthalate, which are stacked from a feed side to a permeate side. Also, as the permeate-side channel material, a knitted fabric member referred to as tricot, which is finer in mesh than the feed-side channel material, has been used for the purposes of preventing the separation membrane from sinking and of forming a permeate-side flow channel.
In recent years, from increased demands for reduction in cost of fresh water production, membrane elements having higher performance have been required. For example, in order to improve separation performance of the separation membrane elements and to increase the permeated fluid amount per unit time, improvements in performance of separation membrane element members such as channel materials have been proposed.
Specifically, Patent Document 1 proposes a separation membrane element having as a permeate-side channel material a sheet embossed with an uneven pattern. Patent Document 2 proposes a separation membrane element requiring neither a feed-side channel material such as a net nor a permeate-side channel material such as tricot by arranging a channel material formed of an elastomer called a vane on a separation membrane. Further, Patent Document 3 proposes a separation membrane element having a channel material with yarns arranged on a nonwoven fabric.